1. Field of the Invention
The present invention relates to a catalyst based on molybdenum and its use in the isomerization of n-paraffins.
2. Description of the Background
More specifically, the present invention relates to a catalyst based on molibden and silicon and its use in the isomerization reaction of n-paraffins with a number of carbon atoms higher than or equal to 5, preferably between 5 and 40.
The use of catalysts based on molibden for the isomerization of n-paraffins is known in literature. In particular, as described in xe2x80x9cProceeding of the 10th International Congress on Catalysis, Budapest, 1992, 955, the catalytic activity of these materials is linked to the formation of molibden carbides or oxycarbides which, with respect to the starting oxide, have a higher surface area which goes from about 4 m2/g (MoO3 at 99.95% of purity) to values which can reach up to about 200 m2/g.
The passage from molybdenum oxide to the catalyst is laborious and can be achieved in various ways, as illustrated hereunder.
The xe2x80x9cJournal of Solid State Chemistryxe2x80x9d, 59, 1985, 332 and 348, describes attacking molybdenum oxide with a mixture of ammonia/hydrogen at 880xc2x0 C. to obtain the corresponding nitride which is then transformed into carbide by treatment with methane/hydrogen at 900xc2x0 C. The carbides obtained with this method have a surface area of 140-180 m2/g.
Another method is described in xe2x80x9cJournal of Catalysisxe2x80x9d, 106, 1987, 125. According to this method, the molybdenum oxide is treated with a stream of methane/hydrogen at an increasing temperature.
In xe2x80x9cJournal of Catalysisxe2x80x9d 112, 1988, 44, the oxide can be previously impregnated with 0.25% by weight of platinum that acts as carburization catalyst, which takes place at an increasing temperature up to 700xc2x0 C. The end solids have an area of about 200 m2/g.
Alternatively, according to what is described in xe2x80x9cJournal of Catalysisxe2x80x9d 117, 1989, 371, the molybdenum oxide can be reduced with hydrogen to the metal phase which is then carburized with CO at 100xc2x0 C. Or, the carburation reaction can be carried out using vapours of MoO3 on activated carbon obtaining materials with a surface area of 100-200 m2/g, as described in European patent 396 475.
The synthesis in situ of molybdenum oxycarbides has recently been described, starting from MoO3 treated at a low temperature (350xc2x0 C.) in a stream of hydrogen/n-octane for 24 hours (xe2x80x9cCatalysis Todayxe2x80x9d, 35, 1997, 51).
The Applicant has now found a new catalytic structure based on molybdenum oxide and silica which can be used as such in the isomerization reaction of n-paraffins raffins without requiring particular pretreatments.
The present invention therefore relates to a catalyst based on molybdenum and silicon having a surface area ranging from 20 to 400 m2/g and a molar ratio Mo/Si greater than 0.2.
A further object of the present invention relates to a catalyst based on molybdenum and silicon prepared by a process comprising:
a) dissolving a soluble molybdenum salt in an aqueous solution containing at least one basic compound selected from ammonium hydroxides having general formula (I):
R1R2R3R4N+OHxe2x88x92xe2x80x83xe2x80x83(I)
xe2x80x83wherein the groups R1-R4, the same or different, represent aliphatic groups containing from 1 to 7 carbon atoms;
b) adding to the solution of step (a) at least one compound of silicon capable of hydrolyzing to SiO2 in such quantities as to give a molar ratio Mo/Si greater than 0.2 and, optionally, an aliphatic alcohol;
c) gelling the mixture thus obtained and calcining the gel obtained in air at a temperature ranging from 500 to 600xc2x0 C.
Any molybdenum salt soluble in water or in a basic environment can be used in the preparation process of the catalyst of the present invention. Practical examples are halogenated derivatives of molybdenum, for example, having the formula MoO2X2 wherein X represents a halogen such as chlorine, bromine or fluorine, or having the formula MoOX4, wherein X represents a chlorine or fluorine atom, molibdic anhydride, molibdic acid and ammonium tetrahydrate heptamolybdate.
The molybdenum salt is dissolved in an aqueous solution of the basic compound having general formula (I). Of these basic products tetrapropylammonium hydroxide is preferred.
When the molybdenum salt has dissolved, the hydrolyzable silicon compound, optionally diluted with an alcohol, is added to the solution. The preferred silicon compound according to the present invention is silicon tetra-alkyl orthosilicate in which the alkyl group contains from 1 to 4 carbon atoms such as, for example, tetra-ethyl orthosilicate.
The alcohol is preferably selected from aliphatic alcohols, in particular C2-C6 alkyl monoalcohols.
The preparation of the gelifiable solution based on molybdenum, steps (a) and (b), substantially takes place at room temperature, dosing the reaction ingredients so that they respect the following molar ratios:
Mo/Si greater than 0.2;
OHxe2x88x92/(Si+Mo) greater than 0.1;
H2O/(Si+Mo) greater than 5;
Alcohol/H2O between 0 and 20.
More specifically, the ingredients are preferably dosed so as to obtain the following molar ratios:
Mo/Si between 1 and 100;
OHxe2x88x92/(Si+Mo) between 0.2 and 5;
H2O/(Si+Mo) between 10 and 100;
Alcohol/H2O between 0.5 and 2.
When the reaction mixture has been prepared, the gelation phase begins. This can be carried out at room temperature or at a temperature ranging from room values to 100xc2x0 C.
The gelation may require times ranging from a few minutes to several hours (even up to 100) and can take place both under stirring and under static conditions. It leads to the formation of a homogeneous gel which may be transparent or opaque. The formation of supernatant phases has never been observed.
At the end of the gelation phase, the gel produced is dried at 100xc2x0 C. for a few hours and is then calcined in air at 500-600xc2x0 C.
The catalyst of the present invention appears as a solid having a surface area ranging from 20 to 400 m2/g, a pore volume ranging from 0.5 to 1 cm3/g, with distribution centred in the mesopore region.
The catalyst of the present invention is useful in the isomerization reaction of n-paraffins, in particular n-paraffins with a number of carbon atoms higher than or equal to 5, preferably between 5 and 40.
A further object of the present invention therefore relates to a process for the isomerization of n-paraffins characterized in that the isomerization reaction is carried out in the presence of a catalyst prepared by a process comprising:
a) dissolving a soluble molybdenum salt in an aqueous solution containing at least one basic compound selected from ammonium hydroxides having general formula (I):
xe2x80x83R1R2R3R4N+OHxe2x88x92xe2x80x83xe2x80x83(I)
xe2x80x83wherein the groups R1-R4, the same or different, represent aliphatic groups containing from 1 to 7 carbon atoms;
b) adding to the solution of step (a) at least one compound of silicon capable of hydrolyzing to SiO2 and, optionally, an aliphatic alcohol;
c) gelling the mixture thus obtained and calcining the gel obtained in air at a temperature ranging from 500 to 600xc2x0 C.
The preferred catalyst for the isomerization reaction is the catalyst based on molybdenum and silicon having a surface area ranging from 20 to 400 m2/g and a molar ratio Mo/Si greater than 0.2.
The isomerization of n-paraffins can be carried out in any type of reactor. It is preferable, however, to operate with fixed-bed or fluid-bed reactors, either in continuous or batch.
The isomerization reaction is carried out in the presence of hydrogen, at a temperature ranging from 200 to 550xc2x0 C., preferably between 250 and 450xc2x0 C., and at a hydrogen pressure ranging from atmospheric pressure to 10 MPa, preferably from 2 to 6 MPa.